1. Field of the Invention
The present invention relates to a waste heat recovery boiler system having a plurality of boilers which generate steam by utilizing heat from a waste gas of a high temperature and which supply the steam to a common system which operates with steam. The invention is also concerned with a method of operating such a waste heat recovery boiler system.
2. Description of the Prior Art
Japanese Patent Unexamined Publication No. 59-68503 discloses an electrical power generating combined plant with a typical waste heat recovery boiler system with the power generating combined plant having plurality of combinations of gas turbine and waste heat recovery boilers capable of generating steam by the heat recovered from the hot exhaust gas of the gas turbines, and a steam turbine supplied with the steam commonly from the boilers. In the disclosed arrangement if only one waste heat recovery boiler is used to receive exhaust gas from a plurality of gas turbines, the operation of the system may seriously be impaired due to a reverse flow of the exhaust gas which may take place when different gas turbines are operated at different load levels. In order to avoid this problem, it is necessary to take an expensive countermeasure. On the other hand, supply of steam from a plurality of boilers to a common steam turbine offers an advantage of improved plant efficiency as a result of an increase in the size of the steam turbine.
There is a trend for higher temperature waste gas (exhaust gas from gas turbine or waste gas from a blast furnace of iron works) as the heat source of waste heat recovery boiler system. It is said that, when the temperature of the waste gas is high, a higher heat recovery efficiency can be obtained by using a combined plant of reheating type. Therefore, in current waste heat recovery systems, the waste heat recovery boiler includes a main steam generator for generating main steam and a reheater. On the other hand, steam turbine, which is the system which utilizes steam, is composed of a plurality of stages including at least a high-pressure steam turbine and a low-pressure steam turbine. The main steam is introduced into and expanded through the high-pressure turbine and the steam from the high-pressure turbine is introduced into the reheater and the steam generated by the reheater is introduced into the low-pressure or reheat steam turbine so as to drive it.
The combined plant of the reheating type is actually constructed as a multiple-type unit in which a plurality of waste heat recovery boilers and a common steam turbine are arranged such that streams of main steam from the main steam generators of the boilers and the streams of reheat steam from the reheaters of the boilers are respectively joined and then introduced to the respective stages of the steam turbine. This system therefore encounters the following technical problems to be solved.
Referring first to the reheating system, the steam from the high-pressure steam turbine is introduced in to the reheaters of the respective boilers so that reheated steam is generated in each reheater. Thus, the steam from the high-pressure steam turbine is distributed to a plurality of reheaters and the flow rates of the steam flowing into the reheaters are determined by the pressure drop or resistance across the respective reheaters. Actually, however, the reheating system is designed such that the reheaters produce the same flow resistance so that the flow rates of the steam introduced into the reheaters are substantially equalized. On the other hand, the rates of supply of the waste gases from the sources such as gas turbines to the respective waste heat recovery boilers are not always equal. For example, it is often experienced that one of the waste heat recovery boilers receives the waste gas at the rated flow rate, while another boiler receives the waste gas at a smaller rate, e.g., 50%, of the rated flow rate. Such condition tends to occur when the level of the load imposed on the whole plant is changed or when the plant is being started or stopped. Consequently, heat is supplied at different rates to different reheaters, though the reheaters are supplied with steam from the high-pressure steam turbine at an equal rate, resulting in different conditions of reheat steam generated by different reheaters. Such different steam conditions may cause a thermal stress to be generated in the portion of a reheat steam line at which flows of hot reheat steam from all reheaters join into a common pipe, causing a risk of rupturing of the reheat steam line due to the thermal stress.
When one of the heat sources, e.g., a gas turbine, and the associated waste heat recovery boiler are being started while another waste heat recovery boiler is operating, the time required for starting is prolonged due to the fact that the steam from the high-pressure steam turbine flows through the reheater of this boiler at a rate greater than the rate which corresponds to the load on the newly started gas turbine. On the other hand, the flow rate of the steam through the reheater of the operating waste heat recovery boiler is reduced to a level below that corresponding to the load on the gas turbine. Consequently, the operation of the whole plant is disturbed tremendously.
Problems are not serious in the case of the main steam system, because the water levels in the drums of the respective main steam generators are independently controllable so as to eliminate any large difference of the conditions of the main steam generated by these main steam generators. Namely, when the rate of generation of main steam in a main steam generator is increased due to an increase in the flow rate of the waste gas supplied thereto, the water level in the drum of the main steam generator is lowered and, when the water level is reduced to the lower limit level, a boiler control operates to increase the rate of supply of feedwater so as to recover the normal water level in the drum. The increased rate of supply of the feedwater naturally reduces the rate of generation of the steam. Conversely, when the rate of generation of main steam in the main steam generator is decreased as a result of reduction in the flow rate of the waste gas supplied to the main steam generator, the water level in the water drum rises and, when the upper limit level is reached, the boiler control serves to reduce the rate of supply of the feedwater so as to recover the normal water level, thus increasing the rate of generation of the main steam.
It is therefore expected that any variation in the waste gas flow rate does not cause serious difference in the conditions of the main steam generated by the main steam generators. Thus, the thermal stress and the condition of operation of the plant are not seriously affected by difference in the waste gas flow rate between different waste heat recovery boilers.